A typical double-conversion superheterodyne receiver has a configuration shown in FIG. 17. Specifically, a signal received by an antenna 61 is supplied to an antenna tuned circuit 201 to obtain a target reception frequency fRX which is then supplied to a first mixer circuit 203 via a high-frequency amplifier 202. The received signal is subjected to frequency-conversion by the first mixer circuit 203 into a first intermediate frequency fIF1 by a first local oscillation signal supplied from a first local oscillator circuit 205. The received signal is further supplied to a second mixer circuit 206 via a first intermediate-frequency amplifier 204. The signal is subjected to frequency-conversion by the second mixer circuit 206 into a second intermediate frequency fIF2 by a second local oscillation signal supplied from a second local oscillator circuit 208. For example, when the first intermediate frequency fIF1 is 58 MHz and the second intermediate frequency fIF2 is 450 kHz, the reception bandwidth can range from 150 kHz to 30 MHz if the first local oscillation frequency fLO1 changes in a range between 58.6 MHz and 88.45 MHz. The signal is further supplied to a second intermediate frequency amplifier 207 and then to a detector 209.
In superheterodyne receivers, provided that:
fRX: reception frequency (desired frequency at which a signal is received)
fLO: local oscillation frequency
fIF: intermediate frequency, then,fRX=fLO−fIF  (1)orfRX=fLO+fIF  (2)The reception frequency fRX is defined by the local oscillation frequency fLO.
Thus, the tuning frequency fTN of an antenna tuned circuit must be deviated accurately by the intermediate frequency fIF from the local oscillation frequency fLO. If the tuning frequency fTN contains an error, the level of the received signal of the frequency fRX is low, thereby reducing the reception sensitivity. The error between the local oscillation frequency fLO and the tuning frequency fTN is called “tracking error”.
In the receiver shown in FIG. 17, practically, a ferrite bar antenna is used to cover the long-wave band (150 kHz to 520 kHz) and the medium-wave band (522 kHz to 1800 kHz), while an external antenna is used to cover the short-wave band (1.8 MHz to 30 MHz). Therefore, an antenna tuned circuit for use in the long- and medium-wave bands, and an antenna tuned circuit for use in the short-wave band are separately required.
Even when an antenna tuned circuit for use in the short-wave band is provided, however, as discussed above, the short-wave band is as broad as 1.8 MHz to 30 MHz, and the tracking error should also be considered. In a practical receiver, thus, an antenna tuned circuit for use in the short-wave band is implemented as a band-pass filter which uses as pass bands frequency bands into which the short-wave band is divided. The antenna tuned circuit for use in the short-wave band is therefore an uninterlocked circuit.
If the antenna tuned circuit is an uninterlocked circuit, however, both the target frequency and the remaining frequencies are supplied to the following stages, resulting in an unsatisfactory interference characteristic. Furthermore, the following high-frequency amplifier must be formed of a special low-noise junction FET for the purpose of noise reduction, which cannot be incorporated together with the other circuits into an IC, thus preventing simplification in assembly or packaging.
Since the first local oscillation frequency in the short-wave band is as high as 60.25 MHz to 88.45 MHz, if a synthesizer receiver is used in which a first local oscillator circuit is formed of a VCO in a PLL, low phase noise cannot be achieved in the first local oscillation signal. In particular, if a frequency step in the reception frequency band is set small, the PLL does not have a wide loop bandwidth, and it is more difficult to improve the characteristic.
Meanwhile, in the antenna tuned circuit for use in the long- and medium-wave bands, if a padding capacitor (a capacitor for frequency adjustment) is adjusted so as to minimize the tracking error in the medium-wave band, the tracking error increases in the long-wave band. Conversely, if the padding capacitor is adjusted so as to minimize the tracking error in the long-wave band, the tracking error increases in the medium-wave band.
Therefore, due to the tracking error, the reception sensitivity is reduced in the long or the medium-wave band. If the antenna tuned circuit for use in the long- and medium-wave bands is an uninterlocked circuit in order to avoid such a reduction of the reception sensitivity, the above-described problems occur.
Another conceivable receiver includes an antenna tuned circuit for use in the long- and medium-wave bands, and a non-volatile memory having antenna tuning data stored therein, such that portion of the data which corresponds to the reception frequency is subjected to D/A conversion and the result is supplied to the antenna tuned circuit. Such a receiver allows the tuning frequency fTN of the antenna tuned circuit to be controlled with accuracy to the reception frequency fRX defined by the local oscillation frequency fLO, resulting in no tracking error.
In this case, however, it is necessary to adjust the tuning frequency fTN for each receiver and to store the resultant data in the non-volatile memory, which wastes much time and labor, leading to increasing cost.
The present invention is intended to overcome the foregoing problems.